1. Field of the Invention
The present invention relates to a small-size, low-height antenna device that is suitably used for an automobile antenna or a portable antenna.
2. Description of the Related Art
Conventionally, as an antenna device which can be suitably implemented as a small-size, low-height antenna device, a T-shaped monopole antenna comprising a band-shaped conductor which is provided on a grounding conductor, and whose lower end is connected to a feeding circuit; and an upper conductor which is arranged above the grounding conductor so as to be substantially parallel and opposite to the grounding conductor and whose center is connected to an upper end of the band-shaped conductor, has been suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-133843 (page 3, FIG. 1). In such a monopole antenna, the upper conductor is disposed on a capacitor region having a large voltage change, a capacitance value becomes high, and an electric field is reduced. As a result, the height of the entire antenna can be reduced to facilitate the effort in decreasing the overall size of antennas. By supplying a power to the band-shaped conductor, it is possible to operate the upper conductor as a radiating element.
In addition, as the reduction in size of antenna devices becomes more required, an inverted F-type antenna has been conventionally adopted, which comprises a radiating conductive plate arranged above a grounding conductor so as to be substantially parallel and opposite to the grounding conductor; a feeding conductive plate that extends orthogonally from an outer edge of the radiating conductive plate and is connected to a feeding circuit; and a shorting conductive plate that extends orthogonally from an outer edge of the radiating conductive plate and is connected to the grounding conductor. In such an inverted F-type antenna, by supplying a power to the feeding conductive plate, it is possible to operate the radiating conductive plate to the radiating element, and by suitably selecting a position of forming the shorting conductive plate, impedance mismatching can be easily avoided. Accordingly, the height of the entire antenna can be made still smaller.
However, in automobile antenna devices or portable antenna devices, since the antenna devices are required to be smaller and shorter in size, the above-mentioned T-shaped monopole antenna or inverted F-type antenna device have been widely adopted. Generally, the antenna device has a characteristic that by making the antenna device smaller and shorter in size, a bandwidth capable of being resonated becomes narrower. As a result, when making the above-mentioned conventional T-shaped monopole antenna or inverted F-type antenna smaller and shorter in size, there was a fear that it is impossible to ensure a predetermined bandwidth. Here, the bandwidth is in the frequency range in which a return loss (reflection attenuation quantity) is not more than −10 dB. But, the antenna device must ensure a bandwidth wider than the bandwidth of a use frequency. For this reason, making the antenna smaller and shorter in size becomes a difficult process.